DETERMINING EMPIRICAL AND MOLECULAR FORMULA
The empirical formula is the simplest formula for a compound. A molecular
formula is the same as or a multiple of the empirical formula, and is based on the
actual number of atoms of each type in the compound. For example, if the
empirical formula of a compound is C3H8, its molecular formula may be C3H8,
C6H16, C9H24, etc.
An empirical formula is often calculated from elemental composition data. The
weight percentage of each of the elements present in the compound is given by
this elemental composition.
Let's determine the empirical formula for a compound with the following
elemental composition:
40.00% C, 6.72% H, 53.29% O.
The first step will be to assume exactly 100 g of this substance. This means in 100
g of this compound, 40.00 g will be due to carbon, 6.72 g will be due to hydrogen,
and 53.29 g will be due to oxygen. In order to compare these quantities, they must
be expressed in terms of moles.
So the second task will be to convert each of these masses to moles, using their
respective atomic weights:
Take notice that since the composition data was given to four significant figures,
the atomic weights used in the calculation were to at least four significant figures.
Using fewer significant figures may actually lead to an erroneous formula.
Now that the moles of each element are known, a comparison between the
elements can be made to determine the empirical formula.
(Step 3)This is achieved by dividing through each of the mole quantities by which
ever mole quantity is the smallest number of moles. In this example, the smallest
mole quantity is either the moles of carbon or moles of oxygen (3.331 mol):
→
Possible extra step**If some results from Step 3 are far from integers (end in .25,
.333, or .5), multiply each ratio by a common factor that converts all molar
amounts to integers or near-integers (within .01).
The ratio of C:H:O has been found to be 1:2:1, thus the empirical formula is:
CH2O. Again, as a reminder, this is the simplest formula for the compound, and
not necessarily the molecular formula. Suppose we know that the molecular
weight of this compound is 180 g/mol. With this information, the molecular
formula may be determined. The formula weight of the empirical formula is 30
g/mol. Divide the molecular weight by the empirical formula weight to find a
multiple:
Molecular mass/ Empirical mass =
The molecular formula is a multiple of 6 times the empirical formula:
C(1 x 6) H(2 x 6) O(1 x 6) which becomes C6H12O6